A number of techniques have been used to measure the position of large rotating objects such as telescope domes. The most common approach has been a mechanical coupling between a conventional rotary position encoder and the moving surface. Thus, telescope dome position has traditionally been encoded by mechanically coupling a rotary encoder to the edge of the dome.
For example, a pinch roller has been pressed against an inner circumference of the dome, to rotate as the dome revolved However, if this pinch roller slipped, large errors in position measurement resulted. An alternative approach to the problem has been to attach a chain to the inner circumference of the dome, the chain then driving a sprocket In either case, the rotating axle of the pinch roller or the sprocket has been used to turn the shaft of a conventional rotary position encoder.
Both incremental and absolute position encoders have been used. When an absolute position encoder was used, a custom-made gear reduction was usually required, so that one rotation of the dome would produce one rotation of the absolute position encoder. Such a custom gear reduction often proved to be more expensive than the absolute position encoder itself. It also tended to introduce errors in position measurement, due to mechanical backlash, and to inexact gearing. When the gear reduction was not exact, the encoder incurred a small incremental error on each rotation; then when the dome was rotated repeatedly in the same direction, this error would accumulate and grow unacceptably large.
While these mechanical couplings sometimes worked well for many years, they could become and eventually became unreliable, especially after decades of mechanical wear. Older domes that shook, wobbled, and nutated as they revolved caused these parts to wear out even faster. For example, the 100-year-old dome on the 1-meter Nickel Telescope and the 30-year-old dome on the 3-meter Shane Telescope, both at Lick Observatory, have suffered from unreliable dome pointing, caused by worn mechanical couplings between the position encoder and the dome.
If an incremental position encoder is used, then some other mechanism is required to establish an initial absolute position. This has often been accomplished by a switch which was tripped whenever the dome was rotated to an index or reference position. However, for a large dome, it can take a considerable period of time to rotate the dome to its initializing position.
In either case, while one can easily obtain conventional position encoders (either absolute or incremental) that are by themselves relatively accurate, reliable, and inexpensive, the accuracy, reliability, and economy of the position measurement that is ultimately achieved is often compromised by the inaccuracy, unreliability, and expense of the mechanical coupling between the encoder and the dome.
Even if an inexpensive and reliable mechanical coupling could be found, currently available rotary encoders do not utilize internal redundancy in their encoding of position. These encoders can and do malfunction, but they provide no mechanism for automatically detecting such errors. As a result, there is often considerable delay in determining that a malfunction has occurred. In the meantime, valuable observing time is lost when the dome does not point in the proper direction.
Other techniques have been employed, such as having a series of separate switches spaced at regular intervals around the circumference of the stationary part of the dome building; the separate switches were then tripped by a single detent on the moving part of the dome. This method required a tremendous number of switches and wires, and so it was quite costly and complex to maintain. To provide angular resolution comparable to that provided by the present invention, even on an average size dome, would require several thousand separate switches. Clearly, this technique does not offer the requisite economy, accuracy, and reliability.
One object of the invention is to solve these problems and to prevent their recurrence.
Because of these problems, various schemes of optical encoding have been tried or considered. All of these schemes involved placing one or more bands of stripes or codes around an inner surface of the rotating portion of the dome. These codes would be sensed by one or more optical sensors attached to the fixed portion of the dome. One such scheme, that of Calvin Delaney in 1979, was a single-track incremental encoder. A similar scheme was used in 1971 for an incremental encoder on the dome of the 40-cm photometric telescope of the Observatoire de Haute Provence, France. Other optical-based schemes that were considered would have used a wider coded band to make a multi-track Gray-code absolute encoder. However, none of these went into general use because of their excessive electronic and mechanical complexity and their susceptibility to skewing errors.
One proposed idea for solving these problems was to use inexpensive commercial fixed-beam bar-code readers to measure the absolute dome position. The idea was to place bar-code symbols around the inside of the rotating part of the dome, with the absolute position at each point directly encoded in the corresponding bar-code symbol. As the dome rotated, the bar-code symbols would be drawn past a fixed-beam bar-code reader, which would decode them and provide the absolute position of the dome.
Unfortunately, standard bar-code symbols cannot be scanned reliably this way. Although such symbols can be scanned at many different speeds and from either direction, the speed and direction during any single scan must remain reasonably constant. Since the dome speed or direction can change at any time, the normal motions of the dome cannot be used reliably to scan standard bar-code symbols.
Another object of the present invention is to overcome the problems that are inherent in prior-art optical encoders.
Another problem is presented by the fact that many domes are out-of-round. For example, each of the domes at both the 1-meter and 3-meter Lick Observatory telescopes is out-of-round by more than one inch, and this eccentricity exceeds the depth of focus of most inexpensive optical sensors. Further, besides being out-of-round, the encoder track surface at the dome of the 1-meter telescope at Lick Observatory exhibits considerable vertical warping. Such out-of-roundness and vertical warping would also complicate any mechanical coupling between such surfaces and a conventional position encoder.
An object of this invention is to overcome problems presented by out-of-round domes and by vertical warping.
A further object is to provide an encoder system with automatic detection of errors.
Other objects and advantages of the invention will appear from the following description.